1. Field of the Invention
The teachings herein relate to characterization of geologic features, and in particular to mapping of groundwater and hydrocarbon depositions using surface deployments of Nuclear Magnetic Resonance (NMR) technology.
2. Description of the Related Art
Geophysical exploration for hydrocarbon deposits commonly involves drilling of wells into the earth's crust. During the drilling process, or after the process, a variety of technologies are deployed within the wellbore to collect data. The data provided offers explorers insight into potential for locating hydrocarbon resources. One example of such technology is that of Nuclear Magnetic Resonance (NMR). Typically, NMR instruments are lowered into a respective wellbore and the data are collected from some depth below the surface. As one might imagine, collecting data in this manner is quite expensive.
Although performing subterranean measurements can provide data not otherwise available, such measurements are not without limitations. For example, performing measurements from within a wellbore provides users with data up to a limited distance away from each wellbore. Accordingly, to obtain accurate depictions of formations for a geographic area, many wells must be drilled. This can be cost prohibitive. Thus, NMR technology has been deployed for surface measurements.
Steam Assisted Gravity Drainage (SAGD) is an enhanced oil recovery technology for heavy crude oil and bitumen. Two parallel horizontal oil wells are drilled in the formation. The upper well is used to inject steam and the lower one is used to collect water that results from the condensation of the injected steam as well as the crude oil or bitumen. The injected steam heats the crude oil or bitumen and lowers its viscosity which allows it to flow down into the lower wellbore. The large density contrast between steam on one side and water/hot heavy crude oil on the other side ensures that steam is not produced at the lower production well. The water and crude oil or bitumen is recovered to the surface by several methods such as natural steam lift where some of the recovered hot water condensate flashes in the riser and lifts the column of fluid to the surface, by gas lift where a gas (usually natural gas) is injected into the riser to lift the column of fluid, or by pumps such as progressive cavity pumps that work well for moving high-viscosity fluids with suspended solids.
The original SAGD wells were drilled horizontally from a tunnel in the limestone underburden, accessed with vertical mineshafts. The concept coincided with development of directional drilling techniques that allowed companies to drill horizontal wells accurately, cheaply and efficiently, to the point that it became hard to justify drilling a conventional vertical well any more. With the low cost of drilling horizontal well pairs, and the very high recovery rates of the SAGD process (up to 60% of the oil in place), SAGD is economically attractive to oil companies. This technology is now being exploited due to increased oil prices. For example, many SAGD projects are in progress in the Canadian oil sands, since this region is home of one of the largest deposits of bitumen in the world.
Ground water present in a production layer should be detected and delineated when planning well trajectories in the SAGD process. It is desirable to reduce a number of wells drilled for the purpose of the detection and delineation.
Accordingly, what are needed are techniques for mapping the ground water. Preferably, the techniques are low cost, rapidly deployable and make use of existing technologies, such as NMR.